Asymmetric joint for connecting carbon electrodes

ABSTRACT

An electrode joint having female end face electrode sections coupled together by a male-threaded nipple wherein the threads adjacent the joint face in the upper electrode section are inactivated by being thinned or removed so as to relieve mechanical and thermal stresses concentrated thereat.

United States Patent Barry C. Stieber;

Herman M. Belz, both of Berea, Ohio 873,139

Oct. 29, 1969 Oct. 12, 1971 Union Carbide Corporation New York, N.Y.

Inventors Appl. No. Filed Patented Assignee ASYMMETRIC JOINT FOR CONNECTING CARBON ELECTRODES 9 Claims, 3 Drawing Figs.

U.S. CI 287/127 E, 13/18 Int. Cl. F16b 21/20 Field oiscarch 287/127,

[56] References Cited UNITED STATES PATENTS 3,016,343 1/1962 Krenzke 287/127 E X 3,173,714 3/1965 Whitwell 287/127 E FOREIGN PATENTS 1,194,249 4/1957 France 287/127 E 1,076,845 3/ 1960 Germany 287/ 127 E Primary Examiner-David J. Williamowsky Assistant Examiner-Wayne L. Shedd Attorneys-Paul A. Rose, Robert C Cummings, Frederick J.

McCarthy, Jr. and Cornelius OBrien ABSTRACT: An electrode joint having female end face electrode sections coupled together by a male-threaded nipple wherein the threads adjacent the joint face in the upper electrode section are inactivated by being thinned or removed so as to relieve mechanical and thermal stresses concentrated thereat.

ASYMMETRIC JOINT FOR CONNECTING CARBON ELECTRODES FIELD OF THE INVENTION This invention relates to electrode joints and more specifically to a means for relieving the high mechanical and thermal stresses usually concentrated about the joint area while simultaneously maintaining socket end face-to-socket end face pressure.

DESCRIPTION OF THE PRIOR ART Carbon electrodes used in electric furnaces are fabricated in various lengths and are consumed in use. In commercial practice a continuous operation of the furnace is required and consequently a means for continually feeding the electrodes into the furnace without interruptions is necessary. One solution for providing a continuous electrode feed is to fabricate electrode sections with suitable threaded sockets at both ends so that a threaded male nipple can be inserted into the socket of one electrode section and then a second electrode section can be screwed on the opposite end of the nipple forming a unitary symmetrical electrode joint. In this manner a new electrode section can be screwed onto a nipple fastened to an electrode section being consumed thus securely joining the electrodes to provide a continuous electrode feed operation.

The employment of cylinder-shaped nipples as electrodesecuring rneam proved acceptable but a subsequent improvement was devised, that being the use of a symmetrically tapered nipple having its maximum diameter at the midsection. The minimum diameter of the nipple when screwed into the electrode socket projects farthest into the electrode body. The thread depth of this tapered nipple is uniform over its entire length with each thread having a loaded flank that carries the force between the nipple and electrode sections.

When employing either the cylindrical-shaped or tapered type male nipple in an electrode joint it was found that the combined stresses due to the torque applied in tightening the joint, electrode train support, tilting of the furnace, falling burden, thermal expansion and the like are primarily concentrated at the flank portion of the threads nearest the end faces of the electrodes. These stresses are frequently sufiicient to cause splitting or cracking and sometimes even total fracture at the electrode joint thereby causing a large segment of the electrode to fall free which contributes to large electrode waste.

'The large stress attributed to thermal expansion is due to the different temperatures which exist in the nipple and electrode sections. Specifically, the nipple usually expands more when subjected to a higher temperature environment than the electrode sections. Thus the temperature difference occurring at the connection joint between the upper and lower electrodes during the operational mode of the furnace is substantial so that the expansion between the electrode sections and the nipple creates tension sufficient to split or crack and even break the connection at the joint area. TO obviate this situation a number of solutions have been tried such as the use of shims in the threaded area between the electrodes and the nipple (US. Pat. No. 2,970,854) and the use of a pitch reservoir for the bonding of the nipple to the electrode sections when the electrode is exposed in a high-temperature environment (US. Pat. No. 2,828,162).

It is the purpose of the present invention to provide an economical electrode joint means that will obviate the splitting and breaking of the nipple and/or electrode sections due to the concentration of thermal and mechanical stresses at the joint area.

SUMMARY OF THE INVENTION This invention relates to carbon electrode joints of either the noncrystalline form of carbon or the crystalline graphite form and specifically to means for preventing joint splitting and/or breaking by inactivating the threads at the midsection vicinity of the nipple and/or the corresponding socket end face section of the upper electrode by thinning or removing them. This effectively relieves the threaded engagement between the nipple and electrode at the joint area and thus alleviates the thermal and mechanical stresses concentrated thereat. In conventional-type electrode joints the stresses en countered during operation of the furnace are sufficient to cause splitting and/or breaking at the joint area thus resulting in large-scale electrode loss. In addition, the extent and severity of the splitting also limits the amount of power that can be used in the furnace.

Providing a symmetrical joint with threaded engagement release recesses in both the upper and lower electrode sections has helped to alleviate the splitting and breaking at the joint area due to concentrated stress thereat. However, in the operational mode of the furnace when the joint is between the electrode holder and the charge and specifically when the consuming concave-shaped arc tip reaches the last threaded engagement common to both the lower electrode and the nipple then the so-called stub portion remnant of the lower electrode becomes electrically ineffective. This results in large electrode loss since the electrode in submerged arc applications is usually consumed in a concave-shape manner so that when the apex of the arc tip reaches the last common threaded engagement area between the nipple and electrode, the positive securement of the lower electrode is obliterated causing the loss of the stub. This stub portion has a substantial length remaining at the circumference of the electrode with its length radially decreasing in a somewhat hyperbolical manner. Thus the amelioration of the electrode-splitting problem was replaced by a significant stub loss.

This invention will compensate for the substantial stub loss while essentially eliminating the splitting and breaking of the electrode joint by inactivating the first thread or threads on the upper electrode section closest the end face and/or the mating thread or threads on the nipple. The lower electrode will then be secured to the nipple by threaded engagement means right up to its end face and consequently the concaveshaped arc tip must reach the end face of the lower electrode before the stub portion falls free. The stub portion in this asymmetric joint construction is drastically smaller than the stub portion obtained from the symmetrical joint recess construction. In addition, since the lower electrode is secured to the nipple by threaded engagement means at its end face then the socket end faceto-socket end face electrical conductivity is maintained right up to the time the stub portion falls free. Increased power can be maintained throughout the operation of the furnace since the electrode end faces are in constant surface contact and do not become inactive which would force the nipple to carry most of the current and in turn further cause nipple expansion resulting in possible splitting and/or breaking. Maintaining maximum power during the operation of the furnace is important where production output varies with power input as in the production of phosphorous, ferroalloys, calcium carbide and the like.

DESCRIPTION OF THE DRAWINGS The invention will become more apparent when described in conjunction with the drawings, in which:

FIG. 1 is a front elevation view of conventional threaded female-type electrode sections coupled together by a threaded nipple.

FIG. 2 is a front elevation view of symmetrical recessed electrode sections coupled together by a threaded nipple.

FIG. 3 is a front elevation view of the invention which shows asymmetrical recessed threaded female-type electrode sections coupled together by a threaded nipple.

Referring to the drawing, FIG. 1 shows female-threaded top electrode 1 coupled to female-threaded lower electrode 2 by symmetrical male nipple 3. When this type of electrode joint is exposed to high-temperature environments, as existing within an electric furnace, the thermal expansion of the electrode sections and the nipple are not identical thus causing mechanical stresses concentrating substantially at the joint area. Splits or cracks 8 resulting therefrom sometime cause the electrode joint sections to be off center thereby concentrating the stresses to one side of the socket. In addition to the radial expansion which causes splitting, axial expansion occurs which may be sufficient at the joint area to relax the socket end faceto-socket end face pressure thus increasing the current flow through the nipple. This thermal effect further increases the expansion of the nipple and further aggravates the problems relative thereto up to a point where actual failure at the joint can occur. The unconsumed portion of the electrode can be substantial and leads to a very inefficient furnace operation and in some cases the undesirable contamination of the charge.

FIG. 2 shows a modified view of an electrode joint provided with end face recesses 13 in both electrode sections 1 1 and 12. This type electrode joint has ameliorated the thermal and mechanical stress concentration at the joint area thus substantially eliminating the splitting or cracking thereat. However since-the lower electrode section is consumed by advancing concave-shaped arc tip 9, then when the apex of the arc tip reaches the last electrode-to-nipple-threaded engagement area, electrode 12 loses all its mechanical securement with the nipple and falls free. This loss of electrode stub 14 is substantial and results in an inefficient furnace operation. In addition, when the arc tip reaches the relief-threaded area, all the current passes through the nipple thereby creating new hot spots in electrode section 11.

FIG. 3 shows a preferred embodiment of the invention wherein the end face of electrode 21 is provided with recess 23 which is substantially concentric with the circumference of electrode 21 and can extend radially outward a maximum of about 1 inch measured from the minor axis of the threaded electrode socket. It is fabricated by either removing or thinning the thread or threads in the vicinity of the end face of electrode 21. If the thread or threads are to be removed then they can be removed up to a radial distance of about 1 inch. On the other hand if the thread or threads are to be thinned then at least the normally loaded flank segment of the thread or threads can be thinned a minimum of 0.002 inch by having the socket pitch diameter increased by a corresponding amount. The overall threaded relief on the electrode may extend axially for a length equal to between about 8 and about 50 percent of the pitch diameter of the nipple. Socket depth in electrode 21 is greater than that of electrode 22 so as to provide equal-threaded engagement in the two electrodes thus equalizing the positive-threaded area between the nipple and each electrode.

In the operational mode of the furnace electrode section 22 will be consumed according to the concave-shaped arc tip 9 and until the apex of the arc tip reaches the end face of this electrode, socket end face-to-socket end face pressure will be maintained. Although stub 25 falls off its loss is substantially less than the loss of stub 14 for the symmetrical recess electrode joint construction. Thus in addition to minimum stub loss, maximum power can be maintained during the operation of the furnace even when the electrode joint is between the electrode holder and the arc tip since the socket end face-tosocket end face pressure is maintained right up to the time the electrode stub falls free.

An alternate embodiment of the invention would comprise thinning or removing the recess-mating threaded area on nipple 24 along with or instead of providing the recess in the upper electrode. The thinning or removing of the threaded area of the nipple would be to the same extent as specified above for the electrode. In either case the socket depth in electrode 21 should be greater than the socket depth in electrode 22 so that the threaded engagement in these electrodes will be equal. However the benefit of this invention can also be achieved if the socket depths of electrodes 21 and 23 are made equal.

EXAMPLE Two female electrode sections coupled together by a conventional-type threaded nipple as shown in FIG. 1 were used in an MW electric furnace for the production of phosphorus. The electrode sections were 1 l0 inches long, 55 inches diameter and had sockets which were 21 inches deep with a pitch diameter of 25% inches. The nipple was 42 inches long and had a pitch diameter of 25% inches. The average tolerable current used during the operation of the furnace was about 62 KA producing about 5.2 tons of phosphorus per hour.

The identical furnace specified above was also used to produce a second supply of phosphorus with the exception that an electrode joint structure as shown in FIG. 3 was employed. The female electrode sections were inches long, 55 inches diameter and had upper and lower sockets which were 19 inches and 25 inches deep, respectively, with a pitch diameter of 25% inches. The thread relief extended from the end face of the top electrode to a depth of 8 inches toward the bottom of the socket and had a pitch diameter one-sixteenth inch greater than that of the remainder of the socket. The nipple was 44 inches long and had a pitch diameter of 25% inches. The threaded area fabricated to mate within the recess area was not thinned. The average tolerable current used during the operation of the furnace was about 68-70 KA producing about 5.7 tons of phosphorus per hour. This was an improvement in the applied tolerable current of about 10 percent over the electrode joint fabricated in accordance with FIG. 1 and resulted in an increased production of phosphorus of about 10 percent.

A tapered nipple may be substituted for the solid cylindrical-type nipple without departing from the concept of this invention.

What is claimed is:

1. In an electrode joint comprising an upper end face electrode section, a lower end face electrode section and a threaded nipple connecting said sections in such a manner to insure upper end face to lower end face engagement, the improvement which comprises having at least one thread in and immediately adjacent to the end face of only the upper electrode section inactivated so as to relieve the threaded engagement between the nipple and only said upper electrode section in the vicinity of the joint thereby alleviating the thermal and mechanical stresses concentrated thereat.

2. The electrode joint as in claim 1 wherein at least one thread adjacent the midsection vicinity of the nipple is inactivatedto correspond with the inactive threaded portion of the upper electrode.

3. The electrode joint as in claim 1 wherein said upper electrode has a radially extending recess at its end face adjacent the midsection vicinity of the nipple.

4. The electrode joint as in claim 1 wherein the upper electrode has at least one of said threads inactivated for an axial length between about 8 percent and about 50 percent of the pitch diameter of the nipple.

5. The electrode joint as in claim 1 wherein at least one of said threads of said upper electrode has at least its normally loaded flank segment thinned a minimum of about 0.002 inch.

6. The electrode joint as in claim 1 wherein the socket depth of said upper electrode extends deeper than the socket depth of the lower electrode so that the positive threaded engagement area between the nipple and each electrode is the same.

7. The electrode joint as in claim 3 wherein the recess of said upper electrode extends axially for a length between about 8 percent and about 50 percent of the pitch diameter of the nipple and extends radially outward a maximum distance of about 1 inch from the minor diameter of the threaded electrode socket.

8. In an electrode joint comprising an upper end face electrode section, a lower end face electrode section and a threaded nipple connecting said sections in such a manner to insure upper end face to lower end face engagement, the improvement which comprises an asymmetric nipple having at least one thread inactivated adjacent only the upper electrode end face and designed to mate with at least one thread within 9. The electrode joint of claim 8 wherein at least one of said 5 threads of the nipple has at least its normally loaded flank segthe upper electrode section for an axial length between about 5 mam thinned a minimum of about 0-002 inch- 8 percent and about 50 percent of the pitch diameter of said 

1. In an electrode joint comprising an upper end face electrode section, a lower end face electrode section and a threaded nipple connecting said sections in such a manner to insure upper end face to lower end face engagement, the improvement which comprises having at least one thread in and immediately adjacent to the end face of only the upper electrode section inactivated so as to relieve the threaded engagement between the nipple and only said upper electrode section in the vicinity of the joint thereby alleviating the thermal and mechanical stresses concentrated thereat.
 2. The electrode joint as in claim 1 wherein at least one thread adjacent the midsection vicinity of the nipple is inactivated to correspond with the inactive threaded portion of the upper electrode.
 3. The electrode joint as in claim 1 wherein said upper electrode has a radially extending recess at its end face adjacent the midsection vicinity of the nipple.
 4. The electrode joint as in claim 1 wherein the upper electrode has at least one of said threads inactivated for an axial length between about 8 percent and about 50 percent of the pitch diameter of the nipple.
 5. The electrode joint as in claim 1 wherein at least one of said threads of said upper electrode has at least its normally loaded flank segment thinned a minimum of about 0.002 inch.
 6. The electrode joint as in claim 1 wherein the socket depth of said upper electrode extends deeper than the socket depth of the lower electrode so that the positive threaded engagement area between the nipple and each electrode is the same.
 7. The electrode joint as in claim 3 wherein the recess of said upper electrode extends axially for a length between about 8 percent and about 50 percent of the pitch diameter of the nipple and extends radially outward a maximum distance of about 1 inch from the minor diameter of the threaded electrode socket.
 8. In an electrode joint comprising an upper end face electrode section, a lower end face electrode section and a threaded nipple connecting said sections in such a manner to insure upper end face to lower end face engagement, the improvement which comprises an asymmetric nipple having at least one thread inactivated adjacent only the upper electrode end face and designed to mate with at least one thread within the upper electrode section for an axial length between about 8 percent and about 50 percent of the pitch diameter of said nipple so as to alleviate the thermal and mechanical stresses concentrated thereat.
 9. The electrode joint of claim 8 wherein at least one of said threads of the nipple has at least its normally loaded flank segment thinned a minimum of about 0.002 inch. 